After a bit over a year of commuting to work by bike, I've started to ride longer distances in the past couple months, and I got a (used, but nice) steal-framed cyclocross bike.

In the past few weeks, I've noticed my lower back hurting more and more. It's right along the spine near the tailbone. I wonder if anyone has any advice about most probable causes. I was thinking it could be any of:

Hard to say without seeing you on your bike, but my guess is that you have a fit issue. I would take your bike into a good shop that does bike fitting. River City, Bike Gallery, Veloce are but a few in town that offer this service.

Going to a shop that does fittings is the best idea. It could be a few things, having someone look at you on yor bike will narrow it down quickly. Your bike may be too small, could be low core strength, or (like many riders) you are not holding your lower back flat.

The guys at River City are specialists at this. It's expensive, but once you get there, you'll see why. They don't just look at you on the bike and make adjustments, they sit and talk with you about where and how you like to ride as well. They're impressive.

I would also consider dumping the backpack and switching to panniers.

Last, if you have insurance, ask your MD, and ask him about posture and potential physical therapy if it's that bad.

Or, try to find a chiropracter that is skilled in lower back pain, and doesn't just "crack 'em and send 'em". This doctor for example deals almost exclusively in accupressure and pressure point releases, plus stretching and other home exercises to help with alignment.

Having said all that, lower back pain is one of the most difficult things to specificially diagnose, and properly treat with precision.

my first mountain-biking experience was a ton of fun, but a couple days afterward, my lower back just started to KILL, and it lasted for a few weeks. i blamed the bumpy terrain, but i've never heard other mountain bikers complain about this problem, so it could have been something else. are you off-road? if yes, do you have good shocks?

one thing that i think helped with my back pain was (much-needed) situps, as my high school gym teacher said that a stronger stomach helps support your back.

I've had that problem when my bars were too low, and I was doubled over too much, especially while climbing. Had to go with risers on the MTB, and riser stems on the road bike. The bars on all my bikes are unfashionably high now, but I feel a lot better.

A week or so after posting this, I woke up one morning and literally couldn't get out of bed because of extreme back pain. When I posted, it was just a dull ache, but that morning, I was completely incapacitated. Nothing like that has ever happened before.

My friends tell me I'm almost 30

Anyway, I've resolved to get fitted and see an MD as suggested above. I'm resisting getting fitted because I don't want anyone to tell me that my bike that I love doesn't fit me. I'm rather tall, so there's a good chance.

In the mean time, stretching and sit-ups are essential. I slowed down for about a week, but yesterday I biked a personal best in terms of distance. In fact, I biked a lot last week and feel pretty much fine as long as I stretch

Not to make this an endorsement free for all, but I think Stephanie at Bike Gallery downtown/Woodstock is doing a fine job, and like Mike Sylvester she has a varied background - fitting, biking, and massage therapy. And she has a good feel for what she was looking at and doesn't 'just' take measurements with fancy devices, which I don't think necessarily results in the best fit.
Just my two cents - as someone who rode frames that were too small because I was too stubborn to get a fitting.